X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2001-06-21
2003-07-08
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S034000
Reexamination Certificate
active
06590959
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to sources of electromagnetic radiation (EMR) that can produce EMR in the extreme ultraviolet (soft X-ray) range of the electromagnetic spectrum. EMR from such a source can be used for microlithography, which is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like.
BACKGROUND OF THE INVENTION
As noted above, a key technique in the manufacture of microelectronic devices such as integrated circuits is microlithography. Most conventional microlithography is performed using deep ultraviolet (DUV) light. The pattern to be transferred is defined on a reticle or mask that is illuminated by a beam of DUV light. A downstream image of the illuminated portion of the reticle is projected (usually with demagnification) by a beam of DUV light onto a suitable substrate (e.g., semiconductor wafer) coated with a resist that is “sensitive” to exposure by the DUV light. Microlithography performed using DUV light still is within the realm of “optical microlithography.”
With ever-increasing miniaturization and density of microelectronic devices, the need has become acute for a microlithography method offering greater resolution than optical microlithography. In fact, optical microlithography now is being conducted at or very nearly at the diffraction limit of DUV light, which means that substantially greater resolution than currently obtainable is probably not possible with optical microlithography. As a result of this dilemma, considerable research and development effort currently is underway to develop a practical “next generation” microlithography apparatus. Among top contenders are charged-particle-beam microlithography and “extreme ultraviolet” (also termed “EUV” or “soft X-ray”) microlithography. The EUV wavelength range receiving the most current attention is 11 to 13 nm.
Unfortunately, EUV light and EMR of neighboring wavelengths are strongly absorbed by most known substances, and no optical materials are currently known that are transmissive to such EMR. Hence, with such EMR as used for microlithography, there is no known way in which to provide a refracting system that can be used for reticle illumination and/or projection of an image onto a substrate. Consequently, illumination-optical systems and projection-optical systems for use in microlithography performed using these short-wavelength EMRs must be made of reflecting optical elements.
Another difficulty with EUV radiation and related short-wavelength EMR is that reflectance of such radiation from ordinary reflective mirrors is extremely low. To obtain maximal reflectance, the mirrors are configured with reflecting surfaces made of a multilayer-film structure. For example, EUV-reflective mirrors have been produced with multilayer reflective films of molybdenum (Mo) and silicon (Si) for reflecting 13-nm EUV light, and multilayer reflective films of Mo and beryllium (Be) for reflecting 11-nm EUV light. However, even with the most efficient mirrors of these types, reflectance of EUV light is at most about 70%. The resulting loss of EMR at each mirror in the illumination-optical system and projection-optical system has led to considerable difficulty in achieving satisfactory imaging performance and throughput.
EUV radiation used in the technologies summarized above typically is produced from a highly specialized source such as an undulator, a laser-plasma source, or a discharge-plasma source. The latter two are attractive because of their relatively small size. In a laser-plasma source, a high-intensity pulsed laser light is converged on a target material to cause the target material to produce a high-temperature plasma from which EUV radiation is emitted. In a discharge-plasma source, the plasma is produced by electrical discharge between electrodes.
An exemplary plasma-focused source (a type of discharge-plasma source) is disclosed in Japan Kôkai Patent document no. Hei 10-319195 and shown in FIG.
8
. The source includes an anode
1
, a cathode
2
, and a base member
3
situated inside a vacuum chamber
8
. The electrodes
1
,
2
are connected to and energized by high-voltage pulses produced by a pulse generator
7
. A working-gas mixture (consisting of a buffer gas and a working gas that produces a desired transition when exposed to an electrical discharge) is introduced into the vacuum chamber
8
via a conduit
10
. Specifically, the working-gas mixture is introduced by the conduit
10
to a space above the base member
3
and between the anode
1
and cathode
2
. The cathode
2
surrounds the anode
1
in the manner of a cylinder. High-voltage pulses from the high-voltage pulse generator
7
are applied across the electrodes
1
,
2
to create a discharge between the electrodes
1
,
2
. The discharge begins on the surface of the base member
3
and produces an “initial” plasma. The initial plasma is formed by ionization of the working gas in the region between the electrodes
1
,
2
and above the base member
3
.
Upon creation of the initial plasma, electrons and ions in the initial plasma move relative to each other under the influence of the electric-field produced by the voltage gradient between the electrodes
1
,
2
, thereby forming a current in the plasma. The current in the plasma, in turn, generates a magnetic field in the plasma. The ions and electrons moving through the plasma interact with the magnetic field and move upward. As a result, the plasma becomes concentrated at the distal end of the anode
1
. The concentrated plasma has elevated temperature and density, sufficient to produce EUV light that radiates from the plasma.
In these sources, the material that actually forms the plasma is material situated at the electrode member excited by the concentrated plasma. Typically, the material includes not only the electrode member itself but also molecules of the working gas situated in the immediate vicinity of the electrode. The wavelength of EMR produced by the plasma corresponds to specific transitions in ions of the electrode member and of the working gas. The plasma region in which the desired EMR is produced is situated substantially within a volume having a diameter of about 1 mm at the distal tip of the electrode
1
. Because plasma production is pulsatile, release of radiation from the plasma is pulsatile. Each pulse of released EMR has a duration in the range of about 0.1 &mgr;s to 1 &mgr;s. By way of example, if the working-gas mixture surrounding the distal end of the electrode
1
contains lithium vapor, then the resulting line spectrum of the produced EUV radiation is about 13.5 nm, which is attributable to the transition in the lithium ions in the plasma.
The amount of EMR produced per pulse by the plasma-focused source of
FIG. 8
is greater than from a laser-plasma light source. Also, with this plasma-focused source, EMR can be produced having a relatively high pulse rate, e.g., of up to several kilohertz. Increasing the pulse rate yields an increase in the net amount of EMR that can be obtained from the source and allows the amount of radiation produced per unit time from the source to be controlled with higher precision.
Japan Kôkai Patent Document No. Hei 11-312638 discloses use of an EUV light source, as described above, in an EUV microlithography apparatus. The optical system downstream of the source is depicted in
FIG. 9
herein, wherein the rays
6
are propagating from the source. The optical elements
11
a
and
11
b
are “fly-eye” (compound) mirrors having respective surfaces such as shown in FIGS.
10
(A) or FIG.
10
(B). Upstream of the mirrors
11
a,
11
b
are other mirrors that collect and collimate the EUV radiation produced by the source. Further with respect to
FIG. 9
, item
12
is a reflective reticle, item
13
is a reticle stage, items
14
a
-
14
f
are mirrors, item
15
is the substrate, and item
16
is a wafer stage. The mirrors
14
a,
14
b,
along with the mirrors
11
a,
11
b
and mirrors situated between the mirror
11
b
and the source, constitute
Kandaka Noriaki
Komatsuda Hideki
Dunn Drew A.
Klarquist & Sparkman, LLP
Nikon Corporation
Thomas Courtney
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